Monday, July 31, 2017

The Aus Jena
Jenapol shearing interference microscope is a rare instrument that was built in
the 1980’s in what was then East Germany.At the time we purchased ours, we were told that there were only about
10 of these in the United States, and we had two of them.

Shearing
interference microscopes work by splitting the image and then recombining the
two images slightly out of phase.The
two images can be both spatially offset, and offset in phase as desired.For evaluation of polymer optical waveguides,
we typically offset the two images 50 microns in the horizontal direction.We then adjusted the optics to produce
fringes with a period of approximately 25 microns. The spacing of the period is
not critical, since the observed offset in the fringes changes
proportionally.We therefore adjusted
the fringe period to best observe the details we were most interested in.The fringe spacing can also be adjusted to
infinity, and produce dark and light areas representing variations in
refractive index.

An optical waveguide viewed under an ordinary microscope
reveals little information about itself.
Below is an image of a multimode 1x2 splitter.

The dimensions can be accurately measured, and any defects
can be observed, but no information regarding the performance characteristics
of the waveguide can be detected.

However, if a waveguide is analyzed using
the Jenapol, a considerable amount of formerly invisible information is
revealed. The delta index of the
waveguide can be accurately measured by measuring the shift in the fringes from
inside the waveguide .The shift is
directly proportional to the index difference.From this, the numerical aperture of the waveguide can be derived

Equally useful, the variations
of index inside the waveguide can be seen.This was very useful in our research to develop planar waveguides with a
graded index.

Below are two waveguides that appeared the same under a
conventional microscope: The left image has fringes with a round profile while
the right has a flat top profile. These profiles provide an accurate indication
of how the waveguide will operate without having to actually test the
waveguide.

The waveguides were
connected to an 850nm source. Both the near field and near fields were analyzed.Below are images of the near fields. These
tests confirmed the predictive accuracy of the Jenapol.

When features are too
small for analyzing with interference fringe patterns, the fringe spacing can
be set to infinity, and the microscope is now sensitive to small variations in
index.

These photos are of a
3 micron thick slice of a polymer film with two singlemode waveguides in
it.In left side photo, the waveguides
are nearly invisible, for the only difference between the waveguides and the
surrounding material is an index difference of .001.However, when viewed in the image contrast
mode, the waveguides are easily visible.

Another application
for the Jenapol is identifying stresses inside a transparent material.The following photos are of a glass fiber
inserted into a polymer.Under a
conventional microscope, it looked perfect. However, the Jenapol revealed
considerable distortion caused by the fiber adhesive diffusing into the
polymer.

The shearing interference microscope can be a
great asset to photonics R&D in ways that were probably not envisioned by
the microscope’s designer over 30 years ago.